Alarm-monitoring system

ABSTRACT

A system for monitoring at a central station the condition, alarm or nonalarm, of a plurality of alarm condition sensors located at a remote station, including interrogation signal means at the central station for interrogating the remote station prior to transmission of each signal of a multisignal train, the interrogation occurring following the expiration of the randomlength delays initiated when signals are received from the remote station, thereby rendering the interrogation process and the resultant signal transmission process aperiodic in nature; a first transmitting means at said remote station responsive to the receipt of interrogation signals for transmitting a first widthmodulated binary signal train when none of the alarm condition sensors is in an alarm condition and a complete inversion thereof when an alarm condition sensor is in an alarm condition; a second transmitting means at the remote station also responsive to the interrogation signals for transmitting a second width-modulated binary signal train when none of the sensors is in an alarm condition, the second width-modulated train having as many signals as there are sensors individually monitored with each sensor associated with a different signal, and for transmitting an inversion of a sensor-associated signal when the associated sensor is in an alarm condition; detecting means at the central station for analyzing the output of the first transmitting means to determine if the first signal train, or a complete inversion thereof, was transmitted and to determine if a sensor-associated signal, or the inversion thereof, was transmitted; and a plurality of alarm indicators each corresponding to a different one of the remote station sensors being individually monitored, each indicator being activated when its respective remote station sensor is in an alarm condition and both an inversion of its respective sensor-associated signal and a complete inversion of the first signal train are detected.

United States Patent [72] inventors John ll. Gallichotte Newtown; Donald E. Hansen, Brooltfield Center; James A. Marquis, Danbury; William J. Shaughnessy, Brookfield Center, all of Conn.

[21] Appl. No. 873,387

[22] Filed Nov. 3,1969

[45] Patented Oct. 5, 1971 [73] Assignee American Standard Inc.

New York, N.Y.

[54] ALARM-MONITORING SYSTEM 18 Claims, 22 Drawing Figs.

[52] US. Cl. 340/408, 1791i .5, 343/65, 340/206, 340/ [50 s 1 int. CL 693;; 25/00, H044 9/ I4 [501 Field of Search 340/2l3,

[56) References Cited UNITED STATES PATENTS 3, l 3 l ,376 4/1964 Duvall 340/408 3, l 7 l ,108 2/1965 Mackeen 340/408 Primary Examiner-John W. Caldwell Assistant Examiner- Robert Jv Mooney Attorney-Wood, Herron & Evans ABSTRACT: A system for monitoring at a central station the condition, alarm or nonalarm, of a plurality of alarm condition sensors located at a remote station, including interrogation signal means at the central station for interrogating the remote station prior to transmission of each signal of a multisignal train, the interrogation occurring following the expiration of the random-length delays initiated when signals are received from the remote station, thereby rendering the interrogation process and the resultant signal transmission process aperiodic in nature; a first transmitting means at said remote station responsive to the receipt of interrogation signals for transmitting a first width-modulated binary signal train when none of the alarm condition sensors is in an alarm condition and a complete inversion thereof when an alarm condition sensor is in an alarm condition; a second transmitting means at the remote station also responsive to the interrogation signals for transmitting a second width-modulated binary signal train when none of the sensors is in an alarm condition, the second width-modulated train having as many signals as there are sensors individually monitored with each sensor associated with a different signal, and for transmitting an inversion of a sensorassociated signal when the associated sensor is in an alarm condition; detecting means at the central station for analyzing the output of the first transmitting means to determine if the first signal train, or a complete inversion thereof, was transmitted and to determine if a sensor-associated signal, or the inversion thereof, was transmitted; and a plurality of alarm indicators each corresponding to a different one of the remote station sensors being individually monitored, each indicator being activated when its respective remote station sensor is in an alarm condition and both an inversion of its respective sensor-associated signal and a complete inversion of the first signal train are detected.

PATENTEU OCT 5 |97| SHEET 02 0F 12 PATENTED [HIT 5 I971 sum 03 or 12 ALARM-MONITORING SYSTEM This invention relates to an alarm-monitoring system and more particularly to an alarm-monitoring system employing a telephone transmission network for interconnecting a remote station, whereat a plurality of alarm condition sensors which are to be individually monitored are located, and a central station, whereat indications of alarm conditions are to be provided, and over which telephone transmission network signals reflecting the state of the alarm condition sensors are transmitted by the remote station to provide an indication at the central station of the existence of an alarm condition.

The general type of alarm--monitoring system to which the invention relates typically includes a remote station such as a bank or department store, which constitutes the area at which unauthorized, or alarm-type, activity is to be monitored; a central station, for example a police station, at which an indication of unauthorized, or alarm, activity is desired; and a telephone transmission network such as a telephone line or microwave transmission link which connects the remote and central stations and over which signals are transmitted to provide the central station with an indication of remote station alarm activity. The remote station has a varying number of alarm condition sensors, such as thermally responsive devices for sensing fires, motion detectors for sensing intrusions, and the like which are activated in response to sensing their respective alarm conditions, such as fires, intrusions, etc. Cooperating with the alarm condition sensors of the remote station is a transmitting device which transmits to the central station suitable signals when an alarm condition sensor is activated in response to the occurrence of its respective alarm condition.

The central station has appropriate indicating devices for indicating at any given time the states of the various remote station alarm condition sensors. Cooperating with the central station indicators is a detector which analyzes the signals transmitted from the remote station and activates the appropriate indicating device when the transmission reflects the fact that a particular remote station sensor has been activated.

An essential requirement of any alarm-monitoring system of the type indicated is that it be as inexpensive and simple as possible consistent with not being easily defeated or compromised. For a variety of unlawful purposes, people often engage in unauthorized activity at a remote-protected area, and obviously attempt to do so without detection by personnel monitoring the central station alarm-indicating equipment. If the alarm-monitoring system, particularly the remote station alarm condition sensors and the associated signal transmitter, are permitted to operate properly and in their contemplated fashion, the unauthorized activity is detected and a signal transmitted to the central station reflecting the existence of an alarm condition.

One seeking to engage in unauthorized activity without detection has the choice of either disabling the alarm condition sensor, such as a motion detector, with the result that the remote station alarm condition sensor never responds to the unauthorized activity, or manipulate the remote station transmitter such that even though the remote station alarm condition sensor detects the unauthorized activity, a normal or nonalarm signal is transmitted. Since most alarm condition sensors are inside the area to be protected, and normally cannot be disabled without activating them and as a consequence transmitting an appropriate signal to alert central station personnel, those seeking to engage in unauthorized activity without detection most often attempt to manipulate the signal transmission rather than the sensors. This permits them to engage in unauthorized activity at the protected area, activating one more alarm condition sensors, but yet go undetected at the central station.

One of the forms of transmission manipulation is that which involves monitoring, over a period of time, the signals transmitted from the remote station when unauthorized activity of the type adapted activate the alarm condition sensors is not occurring. By virtue of such signal transmission monitoring, it

is possible to determine the normal or nonalarm signal format, and using appropriate techniques synthesize such signal format for transmission when unauthorized activity is in progress, thereby effectively camouflaging the unauthorized activity.

It has been an objective of this invention to provide an alarm-monitoring system which is relatively simple and inexpensive and yet very difficult to compromise or defeat by signal analysis and synthesis techniques. This objective has been accomplished in accordance with certain of the principles of this invention by providing at the remote station a transmitter which transmits a sequence of width-modulated binary signals under normal nonalarm conditions and a complete inversion thereof under alarm conditions, and providing at the central station interrogation means which interrogates the remote station transmitter at least at the rate of one interrogation signal per transmitted width-modulated binary signal. Such a monitoring system is effectively asynchronous in nature, rendering defeat or compromise of the system by signal synthesis, made possible as a consequence of transmission monitoring, very difficult.

In accordance with a preferred embodiment of the system designed to provide a particularly high degree of resistance to defeat or compromise by signal analysis and synthesis techniques, the duration or interval between the initiation of transmission of successive binary signal transmissions is randomized, for example, by randomly varying the interval between receipt of a transmitted bit at the central station and initiation of the bit interrogation signal. By virtue of this randomination, bit transmission by the remote station occurs at randomly varying intervals, thereby making the bit transmission process aperiodic in nature. In the preferred system wherein initiation of interrogation occurs at random intervals following receipt of transmitted bits, the transmitter at the remote station is not a free-running transmitter, but rather is randomly, or periodically, keyed on a bit-by-bit basis by successive randomly, or aperiodically, occurring interrogation signals from the central station. A potential attacker cannot. therefore, defeat or compromise the system by simply applying to the transmision line or network a coded sequence of voltage or current signals identical to that obtained by moni toring a previous transmission and synthesizing that which was monitored.

Another important advantage of the system of this invention is that since pulse-width modulation is used there is always a positive response from the remote station regardless of whether the transmitted bit is a l or a 0." The system is, therefore, insensitive to propagation delays and the information rate remains at a maximum. The remote station initiates and terminates the response upon receipt of an interrogation signal, the duration of the response determining whether a logical 1" or a logical 0" is to be transmitted. At the central station, once the beginning of the response is received it is only necessary to sample the response at a point in time greater than the width of a logical 0" and less than the width of a logical l to determine whether a logical 0" or a logical l was transmitted.

In contrast, in other systems wherein the presence or absence of a response is used to indicate the binary nature of the transmitted signal, it is necessary to wait a greater period to determine whether the response is a logical 0" or a logical l The waiting period in such systems begins at the start of the interrogation signal and includes at least two variable transmission line delays. Because of the need to include in the waiting period the transmission delays, the information rate is reduced. Thus, the system, by using pulse-width modulation which makes it insensitive to propagation or transmission line delays, has a greater information rate than comparable systems not using pulse-width modulation.

In a preferred embodiment of the invention, wherein it is desirable to individually monitor the condition of each of a plurality of alarm condition sensors, the remote station trans mitting means is adapted to transmit a first width-modulated binary-signal-sequence or an inversion thereof depending on whether or not none or at least one of the alarm condition sensors has been activated, and a second width-modulated binary signal sequence having as many signals as there are signal devices being individually monitored, the second binary signal sequence being transmitted in a normal form in the absence of an activated alarm condition sensor and with inversion of a sensor-associated signal upon activation of the associated sensor. In accordance with this embodiment, if no alann condition exists at the remote station, the first and second binary signal sequences are transmitted without inversion, whereas if an alarm condition does exist, the first signal sequence is completely inverted as well as that binary signal of the second signal sequence associated with the activated sensor. By virtue of this embodiment, wherein a second signal sequence is used having as many signals as there are alarm condition sensors individually monitored it is possible to extend the advantages of a single-sensor system to a system having a multiplicity of sensors and yet increase the complexity of the system only to the extent needed to process an additional binary signal for each alarm condition sensor being added.

Furthermore, by requiring inversion of all bits of the first signal sequence as well as inversion of one bit of the second signal sequence, it is possible to detect a number of circuit fault conditions which otherwise would go undetected. For example, it is possible to detect the inversion of a sensor-associated bit if the second signal train unaccompanied b a complete inversion of the first signal sequence, or vice versa, as well as detect inversion of at least one but less than all of the signals of the first sequence. Detection of any one of these three partial inversion conditions, which indicate the existence of a circuit fault condition, require little in the way of circuitry in addition to that already needed to detect complete inversion of the first signal sequence and inversion of a sensorassociated binary signal of the second signal sequence.

To facilitate synchronization of the remote station transmitter and central station signal analyzer on a signal train by signal train basis, binary counters are provided at the remote and central stations for effecting the code transmitting and analyzing functions. The binary counter at the remote station is incremented each time an interrogation signal is received, being reset upon receipt of the interrogation signal following transmission of the last signal train bit and not effective to again transmit the first bit of the signal train until the next interrogation signal is received. The counter at the central station is incremented in response to the transmission of the interrogation signal, but is automatically reset when an interrogation signal fails to produce a response. Thus, the interrogation signal following transmission and receipt of the last bit of a signal train is effective to both directly reset the remote station counter, as well as directly reset the central station counter, insuring synchronization between the counters on a signal train by signal train basis. Should the counters become unsynchronized due to a temporary malfunction or receipt of a spurious signal, the unsynchronized condition can only exist for a single signal train transmission cycle.

The use of counters at the remote and central stations for generating the bits to be transmitted and analyzing the transmitted bits, in addition to facilitating counter synchronization on a signal train by signal train basis, is also valuable by virtue of the ease with which it permits the counters to be modified to transmit and analyze different binary signal sequences. Specifically, it is only necessary to alter in a similar manner the connection of the counter outputs at the respective stations to alter the bit composition of the transmitted signal train and facilitate analysis of the altered train upon receipt thereof at the central station. Since, for security reasons, it is often desirable to change the composition of the signal trains, it is advantageous to have a system which permits such changes to be easily made.

These and other advantages and objectives of this invention will become more readily apparent from a detailed discussion of a preferred embodiment of the invention taken in conjunction with a description of the drawings in which:

FIG. 1A is a schematic circuit, in block diagram format, of the remote station;

FIGS. 18 and 1C are collectively a schematic circuit, in block diagram format, of the central station;

FIG. 1D is a layout depicting the relationship of FIGS. lA-IC;

FIG. 2 is an illustrative example of a normal binary signal sequence;

FIG. 3 is an illustrative example of an alarm condition signal sequence;

FIG. 4 is a plot of the energization state versus time of the transmission line for two successive bit transmission cycles in which a logical l" signal and a logical 0 signal, respectively, are transmitted;

FIGS. 5-8 are plots of energization versus time for relays K2, K3, K4 and Kl, respectively, during transmission of a logical 1';

FIGS. 9A and 9B are collectively a detailed circuit diagram of a preferred embodiment of the remote station;

FIG. 9C is a layout depicting the relationship of FIGS. 9A and 98',

FIGS. 10A-l0F are collectively a detailed circuit diagram of a preferred embodiment of the central station; and

FIG. 106 is a layout depicting the relationship of FIGS.

l0A-l0F.

FIG. 11 is a schematic circuit diagram of a modified transmission system which facilitates changing the transmitted message from the central station.

The alarm-monitoring system of this invention, as shown best on FIGS. lA-IC includes a remote station I0 (FIG. IA) and a central station 12 (FIGS. 18 and IC) which are interconnected by suitable transmission means, for example, a telephone line 14. The remote station I0 is provided with at least one, and preferably a group of alarm condition sensors or detectors l5, l6, and 17. The alarm condition detectors or sensors 15-17 are designed to sense alarm conditions of the type for which they are particularly adapted, and upon sensing an alarm close their respectively associated switches l5S-I7S and a common switch 208. The detectors 15-" may be of the type adapted to sense movement in a restricted area, opening of a secured door or window, vibration, heat, interruption of a light beam of the like. The sensors 15-" may each take the same form such as motion detectors located in different rooms of a building or buildings of a multibuilding complex, or alternatively one or more of the sensors may be different in form. Illustrative of the latter arrangement is the use of a motion detector in the public area of a bank, a vibration detector in the bank vault, and a light beam-photocell arrangement at the bank entrance.

To facilitate unrestricted use without activating an alarm of a protected area having located therein alarm condition detectors 15-17, such as is desirable during the day when the protected area is, for example, a bank or department store, suitable means (not shown) are provided for disabling the detectors 15-17. The alarm condition detector-disabling means is under the control of authorized personnel at the protected area, such as the bank or store manger, and is selectively operable to either disable or enable the detectors. When the detectors 15-47 are disabled and enabled, the system is what is termed herein the access" and secure" modes, respectively. The detector disable/enable means constitutes no part of this invention, but rather is a conventional technique currently in use and well within the ordinary skill of one in the art.

To facilitate monitoring the condition, alarm or nonalarm, of the alarm condition detectors 15-17, a message transmitter operable in response to randomized interrogation by the cen tral station and dependent on its information content on the condition of the detectors, is provided at the remote station. and a remote station randomized interrogating and message receiving and monitoring circuit is provided at the central station. The message which the remote station transmits consists of a train of IO binary digits or bits, which are divided into three different groups, or signal sequences. differing from each other from the standpoint of their information-conveying function (FIGS. 2 and 3). The first group of message bits is six bits in length and functions to indicate either the presence or absence of an alarm condition at the remote station and is herein termed the alarm condition code group." The bits of the alann condition code group are designed to have a normal form in the absence of an alarm condition FIG. 2), or altematively a totally inverted form in the presence of an alarm condition (FIG. 3). Under nonnal nonalarrn conditions, the alarm condition code group may comprise for example, the bit train I01 (FIG. 2). Under alarm conditions this group would comprise the totally inverted bit train 010010 (FIG. 3).

The second group of message bits comprise as many hits as there are alarm condition detectors at the remote station 10. In the preferred embodiment there are three alarm condition detectors -17, and therefore there are three bits in the second message bit group (FIGS. 2 and 3). The second message bit group is termed herein the alarm identity code group." Each of the three bits of the alarm identity code group is associated with a different one of the alarm condition detectors 15-17, and is a "0" lfits respectivealarm detector is in the normal or nonalarrn state (FIG. 2) or a 1" if its respective alarm detector is in the abnormal or alarm state (FIG. 3).

The third message bit group comprises a single bit. This bit group, herein termed the "secure/access code group," functions to indicate whether the alarm condition detectors at the remote station are in their enabled or disabled condition placing the system in the secure or access modes, respectively. The secure/access code group contains a 0" bit (FIG. 2) if the system is in the secure condition with the alarm condition detectors 15-17 enabled, or a "l bit (FIG. 3) ifthe system is in the access mode with the alarm condition detectors disabled.

I! is important it note at this point that in accordance with this invention the bit composition of the alarm condition code group is relatively unique for any given remote station in that for an n-bit code there exist 2" possible code groups, each of which are different and potentially available as an alarm condition code group. Stated differently, and so long as there are not more remote stations than different code group permutations, the alarm condition code group associated with a first remote station, such as a bank, is different from the alarm condition code group of a second remote station, such as a department store or another bank. This uniqueness of the alarm condition code groups for different remote stations prevents a person familiar with the alarm condition code group of one remote station, such as a bank where the person is employed, from using this knowledge to defeat or compromise an alarmmonitoring system associated with a second and different remote station, such as another bank. Thus, the terminal unit or message transmitter used at a first remote station cannot be successfully used by random substitution to defeat or comprise the alarm-monitoring system associated with a second and difl'erent remote station.

As noted previously, monitoring of the condition of the alarm condition detectors 15-17 at the remote station is accomplished by remote station transmission of the l0-bit coded message, transmission of the message being in response to randomized interrogation by the central station, and subsequent central station analysis of the received message- Specifically, each bit of the 10-bit message is transmitted in response to a separate, randomly occurring interrogation signal from the central station. This randomized interrogation signal-bit transmission combination gives the alarm-monitoring system of this invention the characteristics of an asynchronous and aperiodic transponding system, that is, the message generated at the remote station is not free running, but rather is accomplished on a bit-by-bit basis in response to successive randomly occurring interrogations by the central station. The asynchronous and aperiodic transponding nature of the system makes compromise or defeat more difficult since an attacker cannot defeat the system by simply applying and identical coded sequence of voltage or current pulses, obtained by I transmission monitoring, to the telephone line which interconnects the remote and central stations.

A further characteristic of message transmission by the remote station is that pulse-width modulation techniques are employed for bit transmission. One specified length pulse is used in transmitting logical 0s" and a second and different specified length pulse is used to transmit logical l 's" (FIG. 4). Thus, regardless of whether a "0 or a "1 is being transmitted a positive response is always obtained from the remote station. This is in contrast to other systems in which the presence or absence of a response from the remote station is used to indicate the logic level of the bit. In such systems the remote station must wait a relatively long and arbitrary time to determine whether a response is forthcoming. This waiting period must begin at the start or the beginning of the interrogation signal and account for two variable transmission line delays. Because of these variable delays, the information rate, that is the number of information-conveying bits transmitted per unit time, is reduced. In the system of this invention wherein pulse-width modulation is utilized to provide a positive response regardless of the logic level of the bit, waiting periods to determine the absence of a response are not required, and hence the information rate is not degraded. In addition, the system is insensitive to propagation delays.

The remote station 10 includes a relay K4 and a relay K3, and associated contacts K4 and K3. The relay K4 is connected in series with the telephone line 14 and adapted to be energized (FIG. 7) to close its normally open contact K4 when the telephone line is energized, which occurs in a manner to be described, when an interrogation pulse is received from the central station. When energized in response to an interrogation pulse the relay K4 and its contact K4 function to initiate, after a suitable delay of approximately 60 milliseconds, transmission of a message bit. The 60-msec. delay between receipt of the interrogation pulse and start of trans mission is necessary to keep the transmission rate below 20 Baud to comply with telephone company tariff regulations on transmission rates. Relay K3 and its contact K3 function to interrupt the electrical continuity of the telephone line 14 for period of either 60msec. or msec. depending upon whether a logical 0" or a logical l bit is to be transmitted in response to the interrogation signal (FIG. 6).

A counter 3, which is pulsed each time an interrogation signal is received, cooperates with five coders 31-35, each of which have their inputs connected to the counter outputs. The combination of the counter 30 and the coders 31-35 provides, on coder output lines 31A-35A, and on a sequential basis, the various bits of the 10-bit message. Specifically, coder output line 31A provides the normal or nonalarm form of the six-bit alarm condition code group. The coders 32, 33. and 34 provide on their respective output lines 32A, 33A, and 34A, and on a sequential basis, the normal or nonalarm form of the alarm identity bits, namely, the bits 0, 0, 0, associated with the detectors 15, 16, and 17. The coder 35 provides on its output line 35A a logical 0" corresponding to the secure mode of operation.

The normal or nonalarm form of the alarm condition code group present on coder output line 31A, the normal nonalarm form of the alarm identity code group present on coder output lines 32A-34A, and the normal secure/access mode group on output line 35A can be inverted to represent the existence of an alarm condition, the identity of the particular alarm condition detector 15-17 activated, and the operation of the system in the access mode, respectively, by switching the position of switches 208, 158-178, and 195, respectively. With the alarm condition code group switch 208 in the position opposite to that shown in FIG. 1A the normal alarm condition code group, l0l I01, present on line 3IA is passed through an inverter 41, and output on a common data line 40 in its inverted form 010010, indicating one of the detectors 15-17 is in an alarm condition. With any one of switches 155, 168, and 178 in a position opposite to that shown in FIG. IA, the normal 0" bit present on its associated line 32A, 33A, and 34A is passed through an inverter 42, 43, or 44 providing on the common data line.40 a logical l bit indicating the particular detector 15-1? in the alarm condition. Withsyvitch in the position opposite to that shown in FIG. 1A, the logical bit output on line 35A designating the secure mode is passed through inverter 45 applying a logical I to the common data line 40 indicating that alarm condition detectors 15, I6, and 17 are in the disabled condition placing the system in the access mode.

Since the alarm condition code group is unique for each remote station, that is, different for different remote stations, the coder 31 operates on the outputs of the counter 30 differently for different remote stations. Modifications of the manner in which the coder 31 operates on the outputs of the counter 30 to produce different alarm condition code groups is accomplished easily by merely changing the manner in which the outputs of the counter stages are interconnected to the coder 3|. The mode of operation of the counter 30 need not be different for each remote station, different alarm condition code groups being obtained by merely altering the codercounter interconnection.

To facilitate response by the remote station to an interrogation signal on the telephone line 14, a flip-flop 47 is provided. The flip-flop 47 has an input line 48 which is grounded upon closure of relay contact K4 when the relay K4 is energized by the interrogation signal which energizes the telephone line (FIGS. 4 and 7). Grounding of the flip-flop 47 input line 48, as a consequence of the interrogation signal, sets the flip-flop, producing an output on line 49. The output signal on line 49 has a twofold purpose. First, it advances the counter 30 making available for transmission, via the common data line 40, the next bit of the message. Secondly, setting of the flip-flop 47 in response to receipt of the interrogation signal provides an input on line 49 to a delay circuit 50, the time delay of which is equal to the desired duration of the interrogation signal.

Responsive to the output on line 51 of the delay circuit 50 is a pulse generator 52. The pulse generator 52, following a predetermined interval from the receipt of the interrogation pulse as determined by the delay circuit 50, and herein named the interrogation period, initiates a pulse which is output on line 53. The pulse output on line 53 of the pulse generator 52, in turn, is input to the pulse-width modulator 22. Depending upon whether a logical 0 or a logical l" is present on the common data line 40, the pulse output on line 53 of the pulse generator 52 is either a pulse of 60-msec. length corresponding to a logical 0 or a pulse of l40-msec. length corresponding to a logical "1." The width-modulated pulse output on line 54 from the pulsewidth modulator 22 energizes the relay K3 (FIG. 6), opening the normally closed relay contact K3 to interrupt the telephone line I4 (FIG. 4). Thus, the beginning of the width-modulated pulse on line 54, which follows the receipt of the interrogation signal by a time period equal to the delay of the delay circuit 50, constitutes the termination of the interrogation signal and the beginning of the response of the remote station to the interrogation signal. The relay K3 remains energized with its normally closed contact K3 in the open state, interrupting the telephone line 14, for a period equal to the length of the width-modulated pulse present on line 54, herein termed the response period. When the pulse on line 54 terminates, relay K3 is deenergized and the relay contact K3 returns to its normally closed condition completing the telephone line circuit. Thus, relay contact K3 is in its open condition interrupting the telephone line for a period equal to the length of the width-modulated pulse which can either by 60 msec. or I40 msec. depending upon whether a logical "0 or a logical l is input to the pulse width modulator 22 on the common data line 40.

An end of pulse detector 56 having its input responsive to the width-modulated pulses on line 54 is provided to generate a reset signal upon completion of transmission of a message bit. The reset signal generated by the end of pulse detector 56 is input to the clear terminal of the flip-flop 47 on line 57. The reset signal on line 57 functions to reset the flip-flop 47 such that it is in a condition to be set to step the counter and activate the delay circuit 50 prior to transmitting the next response when the next successive interrogation signal is received at the remote station. Since relay K4 is again energized upon closing of contact K3 at the end of transmission of the width-modulated message bit, the reset signal on line 57 which occurs at the end of transmission of the width-modulated pulse must be long enough to extend beyond the time of closure of contact K4. If this is not so, when relay contact K4 closes upon reenergization of relay K4 at the end of the widthmodulated bit transmission, flip-flop 47 would be set and initiate a response prior to receipt of the next interrogation signal.

The central station 12 includes two relays K1 and K1. Relay K1 is connected to the telephone lines 14 through a phase inverter 60 such that relay KI is energized and deenergized when the telephone lines are incomplete or deenergized and complete or energized, respectively. A battery 300 or other DC source is connected in series with one of the telephone lines 14 to facilitate energization of the telephone lines. The relay K] has a contact Kl which, when the relay is energized and deenergized, alternatively grounds terminals 62 and 63, respectively, corresponding to periods of telephone line interruption and telephone line completion, respectively (FIGS. 4 and 8). Relay K2 has a normally closed contact K2 which is connected in series in the telephone line. The relay K2 is connected between a source of positive potential 65 and a line 66 responsive to an output of a NAND gate 67 of a rest period circuit 69. The NAND gate 67, in cooperation with a delay circuit 64 which is also part of the delay period timer circuit 68, functions to maintain he relay K2 deenergized and the contact K2 closed, completing the telephone line, for a predetermined interval after receipt of a complete message bit, which period herein is called the delay period, as well as maintain the relay K2 energized and its contact K2 open to interrupt the line for a second random length interval, following termination of the delay period, herein called the rest period. Preferably the delay period and rest period exceed 50 msec., for example, the delay period being approximately 83 msec. and the rest period being approximately 60 msec.-l20 msec., to comply with telephone company tariff regulations limiting transmission rates to 20 Baud and accommodate certain timing circuit tolerances and provide the desired randomness between the initiation of successive bit transmissions.

The central station 12 includes a counter 70 which is similar in construction to the counter 30 of the remote station I0. The counter 70 is stepped or advanced by a signal on its input line 71 each time an interrogation signal is transmitted from the central station. Associated with the counter 70 and responsive to its outputs on lines 72 are a plurality of coder circuits 73, 74, 75, 76, 77 and 78. The coder circuit 73, in cooperation with the counter 70, successively generates on its output line 73A the six bits of the alarm condition code group transmitted by the remote station under normal or nonalarm conditions. Thus, on line 73A appears the pulse train lOl l0l correspond ing to the normal alarm condition code group. In a manner to become apparent hereafter, this output on line 73A is compared with he six-bit alarm condition code group actually received from the remote station to ascertain the existence or nonexistence of an alarm condition at the remote station. T six bits successively generated on coder output line 73A are in synchronization with the receipt of the six bits of the alarm condition code group transmitted by the remote station 10.

The coders 74 and 78 provide on their respective output lines 74A and 78A synchronization. or timing. signals upon receipt of the sixth last bit of the alarm condition code group transmitted by the remote station and the 10th and last bit of the message corresponding to the secure/access code group transmitted by the remote station respectively. The coders 75. 76, and 77 provide on their respective output lines 75A. 76A and 77A logical 0s" which correspond to the normal bits of the alarm identity code group transmitted by the remote station. These eentral-station-generated bits are generated in synchronism with the receipt from the remote station of their associated alarm identity code group bits.

To advance the counter 70 upon initiation of an interrogation transmitted to the remote station, a flip-flop 80 is provided. The flip flop 80 is responsive to terminal 63 which is selectively grounded, setting the flip-flop when relay Kl is deenergized upon completion of the telephone lines 14 at the beginning of the interrogation pulse. Thus, the flip-flop 80 provides on its output lines 71 and 82 complementary signals when the flip-flop is set in response to initiation of the interrogation signal. One of these signals is input to the counter on line 71, advancing the counter and thereby internally generates at the central station the next bit of the message or timing signal, as the case may be.

To demodulate or decode the width-modulated pulses received from the remote station 10 on the telephone line 14 a second flip-flop 85 and a data-sampling pulse generator 86 are provided. The input of the flip-flop 85 is responsive to terminal 62 which is grounded by the relay contact K1 when the relay K1 is energized coincident with opening of the telephone line by the remote station at the beginning of the transmission of a message bit. Grounding of terminal 62 at the beginning of the response period sets the flip-flop 85, producing complementary outputs on the flip-flop output lines 87 and 88. The output from the flip-flop 85, coincident with the beginning of the response period is input to the data-sampling pulse generator 86. The data sampling pulse generator 86, in response to this input, provides a sampling pulse of approximately 1 msec. duration on its output line 89 at a point in time 103 msec. after the starting point or leading edge of the received message bit transmitted from the remote station. The data-sampling output pulse on line 89 is used to sample the output on line 82 from flip-flop 80 to determine whether a l40-msec. pulse corresponding to a logical l" or a 60-msec. pulse corresponding to a logical 0" has been received. As indicated previously,

the flip-flop 80 is set when its input, terminal 63, is grounded by deenergization of relay K1 when the telephone lines close at the beginning of the interrogation period. The flipflop 80 is reset at the end of a response period, that is, upon completion of the transmission of the message bit since its input, terminal 63, is again grounded by the deenergization of relay K1 in response to closing of the telephone lines at the remote stalion. Thus, the flip-flop 80 is reset at a point in time corresponding to receipt of the trailing edge of the transmitted message bit. Since the data-sampling pulse generator 86 produces the data-sampling pulse on line 89, 103 msec. after the leading edge of the transmitted data bit has been received, by sampling the state of the flip-flop 80 it is possible to determine whether a logical 0" or a logical I has been transmitted. [f the flip-flop 80 is still in the set condition indicating that the trailing edge of the transmitted pulse has not yet been received, then the transmitted pulse is 140 msec. long and deemed a logical I. If the flip-flop 80 has been reset by receipt of the trailing edge of the transmitted bit, upon sampling of the flip flop output 80 by the sampling pulse on line 89, it is possible to determine that a short-duration pulse of 60 msec. corresponding to a logical 0" has been transmitted.

A comparator 90 is provided in order to ascertain whether the transmitted message bits of the alarm condition code group corresponds to the normal or nonalarm bit, or to an inversion thereof corresponding to an alarm condition. The comparator 90 is responsive to the normal bits of the alarm condition code group generated at the central station by the counter 70 and the coder 73 which are present on the common coder output line 79. The comparator 90 is also responsive to the output on line 82 of the flip-flop 80 which represents the received message bit. The comparator 90 is also responsive to the data-sampling pulse generator output on line 89. In response to these outputs on lines 79, 82, and 89, the comparator produces on its output line 91 a signal each time a received message bit does not match the corresponding bit internally generated by the counter 70 and the coder 73. Thus, on line 91 a mismatch signal is produced each' time a'bit of the alarm condition code group is inverted at the remote station. If any of of the alarm condition sensors 15-17 has been activated to switch its respective switch 158-178 and the switch 205, the bits of the alarm condition code group are inverted producing six sequential mismatch outputs from the comparator on line 91.

Under certain conditions when a circuit fault exists, either at the remote station or the central station, it is possible to have an inversion of one or more, but less than all, of the bits of the alarm condition code group. To distinguish between a complete inversion of all bits of the alarm condition code group which indicates the presence of a true alarm condition at the remote station, and a partial inversion, i.e. inversion of one or more but less than all of the bits, of the alarm condition code group which corresponds to a circuit fault condition, an alarm condition code group inversion detector circuit and a circuit fault detector 96 are provided. The alarm condition code group inversion detector 95 includes a flip-flop 97 which is connected via an inverter 109 to the output line 91 of the comparator 90. By reason of the inverter 109, an input is provided to the flip-flop 97 each time the transmitted bit and the bit internally generated at the central station match, that is, an input is provided to the flip-flop 97 each time a normal, noninverted bit is received. Thus, the flip-flop 97 is set upon receipt of the first noninverted or normal bit. If no noninverted or normal bits are received, the flip-flop 97 remains reset.

Subsequent to receipt of the sixth and last bit of the alarm condition code group, the status of the flip-flop 97, which records received noninverted bits of the alarm condition code group, is sampled by a NAND gate 101 whose one input is responsive to the output of the flip-flop 97 and whose other input is responsive, via NAND gate 74-3, to the output line 74A of coder 74 which produces a signal upon receipt of the sixth and last bit of the alarm condition code group and upon generation of a data-sampling output on line 09 from the datasampling pulse generator 86. The sampled output of the flipflop 97 made subsequent to receipt of the six bits of the alarm condition code group is stored in a storage flip-flop 98. On output line 99 of storage flip-flop 98 a logical 0" signal is present if one or more of the transmitted bits of the alarm condition code group are normal or noninverted, and a logical 1" is present if none of the received alarm condition code group bits are normal.

The alarm condition code group inversion detector 95 also includes a NAND gate 100. The NAND gate has one input connected to output line 99 of flip-flop 98 on which a logical 1 signal is present if no normal or noninverted bit of the alarm condition code group has been received. The NAND gate 100 also has one input connected to a flip-flop 102, to be described, on whose output line )3 a logical l signal appears if at least one alarm or inverted bit is received. Logical l" signals are present on both lines 99 and 103 if no normal or noninverted signals have been received and if at least one alarm or inverted signal is received, and are efl'ective to produce a logical 0," output on NAND gate output line 104, indicating that a completely inverted alarm condition code group has been received.

The circuit fault detector 96 includes a flip-flop 105 responsive to the output line 91 of the comparator 90. The flipflop 105 is set if at least one inverted alarm condition code group bit is received. The contents of llip-flop 105 are gated by a NAND gate 106 upon receipt of an output on line 744A of NAND gate 74-3 which occurs when all six bits of the alarm condition code group are received and a data-sampling pulse is generated. The gated output of the flip-flop 105 is input to the flip-flop 102 producing on line I03 a logical l signal if at least one inverted or alarm bit is received. The circuit fault detector 96 also includes a NAND gate ")7. The NAND gate 107 is connected to the output of the alarm condition code group inversion detector 95 on line 104. A logical l signal is input to NAND gate 107 on line l04 if a complete alarm condition code group inversion has not been detected. The NAND gate 107 is also responsive to the output of flip-flop 102 on line 103. The NAND gate l07 produces a logical 0" signal on output line l08 indicating a circuit fault condition when logical l signals are input on lines 103 and 104 corresponding to conditions of at least one inverted alarm condition code group bit and the absence of a complete inversion of all alarm condition code group bits. Thus, a logical output signal is provided on line 108 of NAND gate 107 indicating a circuit fault when at least one, but not all, of the six alarm condition code group bits is inverted.

The output on line 108 corresponding to a circuit fault of the type wherein at least one, but less than all, of the bits of the alarm condition code group are inverted is input on line 110A to a circuit fault indicator 114 which functions to provide a visual and/or audible indication to the personnel at the central station.

For the purpose of indicating inversion of one of the alarm identity code group bits when all of the alarm condition code group bits are also inverted, reflecting the existence of an alarm condition at the remote station, alarm identity code group bit inversion detector circuits 112-7, 112-8, and 112-9 are provided. The detector 112-7 includes a NAND gate 113-7 which is responsive to the output line 82 of flip-flop 80 which represents the received message bit, output line 75A of coder 75 on which a signal is produced upon receipt of the seventh message bit, and line 89 on which a data-sampling signal is present, The NAND gate 113-7 provides an output on its output line 116-7 a logical 1" is received as the seventh transmitted message bit, a logical l being received when switch 158 is in the position opposite to that shown in FIG. 1A in response to the sensing of an alarm condition by its associated sensor 15. A NAN D gate 117-7 is provided and has one input connected to output line 116-7 of NAND Gate 113-7, and another input line connected to output line 104 of NAND gate 100 indicating an inversion of the alarm identity code bit and a complete inversion of the alarm condition code group. respectively. The NAND gate 117-7 provides an output on line 118-7 when a complete inversion of the alarm condition code group is present as well as an inversion of the alarm identity code bit corresponding toe alarm sensor which has been activated. The output on line 118-7 is input to a latch circuit 119-7 which is switched and latched, providing a continuous signal on line 120-7 to a visual and/or audible detector 121-7 to provide an indication that remote station alarm condition sensor 15 has been activated.

Detector circuits 1 12-8 and 112-9 are identical in structure and operation to detector circuit 112-7 except that detector circuits 112-8 and 112-9 provide indications that remote station alarm condition sensors 16 and 17 have been activated, respectively. Thus, inputs are provided on lines 120-7, 120-8, and 120-9 to indicators 121-7, 121-8, and 121-9 when the seventh, eighth, and ninth message bits, respectively, of the alarm identity code group are inverted indicating the activation of alarm condition sensors 15, 16, and 17, respectively, and the alarm condition code group has been completely inverted. Accordingly, when one of the detectors 121-7, 121-8, or 121-9 has been activated, personnel at the central station are informed that an alarm condition at the remote station exists, as well as the particular one of the alarm condition sensors which has been activated in response to the alarm condition.

To determine whether the system is in the secure or access mode, a secure/access code group bit inversion detector circuit 125 is included. This circuit 125 includes a NAND gate 126 which is responsive to the data-sampling pulse on line 89, the received bit on line 82, and the output of coder 78 on line 78A. NAND Gate 126 produces either a logical 0" or a logical I output on line 127 depending upon whether the system is in the secure or access mode. The NAND gate output on line 127 is input to an indicator driver circuit 128. [la logical 0" is input to the driver circuit 128 on line 127, indicating that the 10th message bit transmitted is a "0 and hence that the remote station is in the secure mode, an input is provided on driver output line 129 to a secure mode indicator 130, providing central station personnel with an indication that the system is in the secure mode. lfa logical "l" is input to the driver circuit 128 on line 127, indicating that the 10th message bit is a logical l and the remote station therefore is in the access mode, an output is provided on driver circuit output line 131 to an access mode indicator 132, providing central station personnel with an indication that the remote station is in the access mode.

To indicate when all 10 bits of the message have been received, an end of message circuit is provided. The end of message circuit 140 includes a delay circuit 141 the input of which is responsive to output line 82 of flip-flop 80. The delay circuit 141 provides on its output line 142 a logical 1 signal at a point in time approximately 320 msec. after the start of the interrogation period. The output on line 142 is input to a NAND gate 143, the other input of which is connected to output line 88 of flip-flop 85. The NAND gate 143 provides on output line 144 a logical "0" reset signal coincident with the generation of the output from delay circuit 141 only when the NAND gate 143 is in the enabled condition by virtue of a logical l input thereto on line 88. It will be recalled that an output signal is provided on line 82 which is the input line to the delay circuit 141 upon initiation of the interrogation period. It will be also recalled that a logical 0" output is provided on line 88 upon receipt of a transmitted bit. Thus, if a transmitted bit is received an flip-flop 85 is set, a logical 0" output is present on line 88 which disables NAND gate 143, preventing NAND gate 143 from providing on its output line 144 a reset signal when the delay circuit 141 produces an output on line 142 approximately 320 msec. after the beginning of the interrogation signal as a consequence of having input thereto on line 82 a signal at the beginning of the interrogation period. However, when an interrogation signal is transmitted subsequent to transmission of the 10th message bit and no response from the remote station is forthcoming, the output of the flip-flop 85 is a logical l which enables gate 143. Therefore, the output of the delay circuit 141 on line 142 is not blocked, and NAND gate 143 gates the signal on line 142, producing on line 144 the reset signal indicating the end of message. The reset or end of message signal on line 144 functions to reset the flip-flops 97 and 105. Additionally, the reset signal on line 144 activates the delay period circuit 68. and in turn the rest period circuit 69 which via NAND gate 67 produces the necessary signals on line 66 to institute the delay and rest periods following the interrogation period succeeding receipt of the last message bit. The reset output on line 144 also resets the counter 70, ensuring synchronism between the central station counter 70 and the remote station counter 30 which was reset upon receipt of the interrogation signal succeeding the transmission of the last message bit.

A telephone line open-circuit detector responsive to terminal 62 is provided. The telephone line open circuit detector 150 provides an output on line 151 to the circuit fault indicator 113 when the telephone line remains in a deenergized condition for an excessively long interval.

A telephone line short circuit detector 153 responsive to output line 88 of the flip-flop 85 is provided to generate a signal on its output line 154 to the circuit fault indicator 113 when the telephone line remains in an energized condition for an excessively long interval.

A regurgitate circuit 157 responsive to terminal 63 is provided for momentarily interrupting the telephone lines should the telephone lines remain energized for an excessive interval. Thus, should the system stall with the telephone lines energized, the regurgitate circuit 157 produces on its output line 158 a. signal which momentarily energizes the relay K2 to interrupt the telephone line.

A sixth bit missing circuit 159 adapted to detect the absence of the sixth message bit is also provided. This circuit provides on its output line 160 a signal to the circuit fault indicator 114 should the sixth bit of a message be missing.

A detailed circuit diagram of a preferred embodiment of the remote station 10 is depicted in FIGS. 9A and 9B. As shown in these figures the remote station includes the transmission line 14 and associated relays K3 and [(4. The relay K4 is connected in the transmission line in parallel with a normally reverse biased diode 200. The relay K4 has a grounded contact K4 alternately connectable between a floating potential terminal 203 and the line 48. When the transmission line is deenergized, the relay K4 is deenergized and the relay contact K4 is in the solid line position shown in FIG. 9A. When the transmission line is energized, energizing the relay K4. the relay contact K4 moves to the dotted-line position shown in FIG. 9A.

The relay K3 is connected between a source of positive potential 205 and ground potential 206 via the emitter-collector path of a normally nonconducting transistor switch 03. A normally reverse biased diode 207 is connected in parallel with the relay K3. The relay K3 has a contact K3 which is connected in the transmission line. The relay K3 is normally deenergized, and when deenergized has its contact K3 in the solid-line position shown in FIG. 9A in which the transmission line circuit is complete. When the relay K3 is energized, the contact K3 moves to the dotted line position shown in H6. 9A. interrupting the electrical continuity of the transmission line. A resistor 201 and capacitor 202 are connected in parallel with the relay contact K3 to prevent spurious signals from being transmitted over the telephone line 14 when the relay contact K3 switches.

The remote station 10 also includes the flip-flop circuit 47. The flip-flop circuit 47 includes a conventional JK flip-flop 210 having an input terminal T and output terminals Q and Q on which are provided complementary output signals. The input terminal T of the flip-flop 210 is connected via a differentiating network 211 to the line 48 which is selectively groundable via relay contact K4. When the relay contact K4 grounds line 48 in response to energization of the relay K4, a negative voltage spike is input to terminal T of flip-flop 210 switching the flip-flop, which is effective to provide at ter: minal Q a positive signal level or logical l and at terminal Q a negative signal level or logical chides a clear terminal CL, which is ellectlve to reset the flip- 1109 when a low signal level or logical is input thereto on line 213. A high signal level (logical "1) and a low signal level (logical "0") are output at terminals 0 and 6 of flip-flop 210 during the interrogation and responsive periods of a message bit transmission cycle. These flip-flop output levels at terminals Q and 0 reverse when flip-flop 210 is reset upon completion of the response period by a low level (logical "0) signal on reset line 213.

The remote station further includes the counter 30 (FIG. 9B). The counter 30 includes four conventional JK flip-flops 214, 215. 216 and 217 which ea c h have aT input terminal and complementary output 0 and Q terminals. The counter flipflops 214-216 have their output terminals 0 and 0 connected to the input terminal T of the successive counter stages 215-217 via resistor and capacitor noise suppression circuits 218-220. The T terminal of the first counter flip-flop state 214 is responsive to the output of the flip-flop 210 via a resistor and capacitor noise suppression circuit 209. With the flip-flop stages 214-216 of the counter 30 connected in the manner indicated. and the first stage 214 responsive to the output of the flip-flop 210, the counter 30 functions as a conventional binary counter.

A four-stage binary counter is capable of developing a count of 16. However. in the present system wherein the counter is used to facilitate transmission of a message having only 10 hits, a counter having the capability of developing a count of 10 is sufficient. Accordingly, additional-circuitry is used in combination with the four stages 214-217 of the counter 30 to reset the counter after it develops a counter of gating input to 10. that is, to reset the counter 30 on a count of l 1; thus. the counter has a capacity of 10. The additional circuitry required to limit the counter 30 to a IO-count capacity includes a flipflop 221. comprising two interconnected NAND Gates 221A and 221B. the inputs of which are responsive to NAND gates 222 and 223. The NAND gate 222 is responsive to various ones of the outputs of the counter flip-flop stages 214-217 and produces on its output line a low level or logical 0" signal on a count of 11 for switching the flip-flop 221 which in turn produces on its output line 224 a low level signal or logical "0" which is input to the clear terminals C1. of the counter flip-flop stages for resetting these stages to provide a counter output of 0000. The NAND gate 223 has four inputs which are responsive to various ones of the outputs of the counter flipflop stages 214-217. The NAND gate 233 functions to produce on its output line a low-level signal or logical "0 when the counter is reset to a count of 0000. This output from NAND gate 223 functions to reset the flip-flop 221 after the counter has been reset to a 0000 count, thereby readying the flip-flop 221 to again reset the counter flip-flop stages 214-217 when the next succeeding count of l l is reached and a reset output is provided by NAND gate 222.

The remote station 10 further includes the alarm condition code group coder 31. The coder 31 includes two NAND gates 225 and 226 each having three inputs connected to various ones of the counter flip-flop stage outputs 214-217. and a NAND gate 227 connected to the outputs of the NAND gates 225 and 226. The coder 31 functions to provide on its output line 31A a train of six bits which comprise the alarm condition code group of the 10-bit message. the six-bit train output on line 31A constituting the normal alarm condition code group depicted in FIG. 2. The output of the coder 31 on line 31A is at a low level or high level depending on whether a logical 0" or a logical -l is being generated.

Associated with the coder 31 is a NAND gate 228 having one of its inputs connected to the output line 31A of the coder 31 and the other of its inputs connected to a source of positive potential 229 via a resistor 230 and to ground potential via switches 155, 168. and 175. in the absence of an alarm condition at the remote station. that is. in the absence of activation of one of the alarm condition detectors 15-17. the switches -175 are in the position shown in FIG. 98. With switches 15S-17S in this position, ground potential is not applied to the NAND gate 228 and the gate is enabled via the positive potential from source 229. With the NAND gate 228 enabled by the positive source 229. the six-bit alarm condition code group output on line 31A from the coder 31 is gated by the NAND gate and output to the pulse-width modulator 22 to be described in detail hereafter. Thus. in the absence of an alarm condition at the remote station. switches 155-175 are in the position shown in FlG. 98 resulting in NAND gate 228 being enabled such that it gates the six bits of the alarm condition code group to the pulse-width modulator on line 232.

An inverter circuit 41 and a switch circuit 208 function to provide. when one of the switches 158-178 is switched from the position shown in FIG. 9 in response to activation of its associated alarm condition sensor 15-17, an alternate transmission path to the pulse-width modulator 22 for the normal alarm condition code group bits output from the coder 31 on line 31A. As noted previously. in the presence of an alarm condition at the remote station. switches 15S-17S are in the position shown in FIG. 9 to enable gate 228 for transmitting the alarm condition code bits output on line 31A from the coder 31 to the pulse-width modulator 22. However. should an alarm condition be detected at the remote station 10 by one of the alarm condition detectors 15-17. the switch 15S-17S associated with the activated alarm condition detector switches from the position shown in FIG. 9. grounding the NAND gate 228. disabling this gate. With NAND gate 228 disabled, the alarm condition code group output on line 31A from the coder 31 is not gated to the pulsewidth modulator on line'232. Instead. the alarm condition code group present on line 31A from the coder 31in its nor- 

1. A system for individually monitoring at a central station the condition, alarm or nonalarm, of a plurality of alarm condition sensors located at a remote station, said system comprising: first transmitting means at said remote station which are responsive to each of said alarm condition sensors for transmitting to said central station a first sequence of widthmodulated binary signals when none of said alarm condition sensors is in an alarm condition and for transmitting to said central station a complete inversion of said first sequence of width-modulated binary signals when one of said sensors is in an alarm condition, second transmitting means at said remote station which are responsive to each of said sensors for transmitting to said central station a second sequence of width-modulated binary signals when none of said sensors is in an alarm condition, said second signal sequence having at least as many signals as there are sensors to be individually monitored with each sensor associated with a different signal, said second transmitting means operative to transmit an inversion of a sensor-associated signal when said associated sensor is in an alarm condition, means at said central station for interrogating said first and second remote station transmitting means prior to the transmission of each width-modulated signal or inversion thereof of said first and second sequences, first detecting means at said central station for analyzing the signals transmitted by said first transmitting means to determine if said complete inversion of first signal sequence thereof was transmitted, second detecting means at said central station for analyzing the signals transmitted by said second transmitting means to determine if said inversion of a sensor-associated signal was transmitted, and a plurality of alarm indicators each corresponding to a different one of said remote station sensors, and each responsive to both said first and second detectors, each said indicator being activated to alert central station personnel that its respective remote station sensor is in an alarm condition in response to both the detection by said first detecting means of the transmission of a complete inversion of said first signal sequence and the detection by said second detecting means of the transmission of an inversion of its respective sensor-associated signal.
 2. The system of claim 1 further including a third detecting means for analyzing the signals transmitted by said first transmitting means to determine if at least one but less than all of said first signals are inverted, and a circuit fault indicator responsive to said third detecting means for altering central station personnel to the existence of a fault condition when at least one but less than all of said first signals are inverted.
 3. The system of claim 1 further including fault-detecting means for determining when only one of said inverted first signal sequence and inverted sensor-associated signal is transmitted.
 4. The system of claim 3 wherein said fault-detecting means includes Exclusive-OR circuit means responsive to both said first and second detecting means, for providing an output when only one of said first and second detecting means detects its respective signal condition.
 5. The system of claim 1 wherein said first detecting means includes means to internally generate at said central station said first signal sequence, means responsive to said internally generated first signal sequence and the signals transmitted by said first transmitting means for providing a first output when at least one of said internally generated signals and said signals transmitted by said first transmitting means does not match and a second output when none of said internally generated signals and said signals transmitted by said first transmitting means match, and coincidence circuit means responsive to said first and second outputs for detecting when both outputs are present and thereby determining the existence of a complete inversion of said first signal sequence.
 6. A system for monitoring at a central station the condition, alarm or nonalarm, of an alarm condition sensor located at a remote station, said system comprising: a code generator at said remote station for generating a normal sequence of width-modulated binary signals, when said alarm condition sensor is in said nonalarm condition, and for generating an alarm sequence of width-modulated binary signals when said sensor is in said alarm condition, interrogation means at said central station for transmitting interrogation signals to said remote station, said interrogation signals being transmitted at least at the rate of one interrogation signal per width-modulated binary signal transmitted by said remote station, transmitting means at said remote station responsive to said interrogation signals for causing said code generator to transmit width-modulated signals of either said alarm or nonalarm sequence, each of said signals of said transmitted sequence being transmitted in response to receipt of a separate interrogation signal, and detector means at said central station for analyzing said transmitted signal sequence to determine the existence of an alarm condition at said remote station.
 7. A system for monitoring at a central station the condition, alarm or nonalarm, of an alarm condition sensor located at a remote station, said system comprising: a telephone line transmission means interconnecting said remote and central stations, said transmission means having a predetermined maximum transmission rate requiring a specified minimum lapsed time interval between the transmission of successive signals, interrogation signal initiation means at said central station for initiating interrogation signals, interrogation signal termination means at said remote station responsive to the receipt of an interrogation signal initiated by said central station for terminating said interrogation signal after a delay following receipt thereof at least equal to said minimum interval, code transmission means at said remote station for transmitting, in response to successive interrogation signals, a sequence of successive width-modulated binary signals at the rate of one binary signal per interrogation signal, said code transmission means including a multistage binary counter having an input responsive to the receipt of an interrogation signal for incrementing said counter and having a plurality of outputs from said stages connectable for facilitating transmission of a first repetitive sequence of width-modulated binary signal sequences, said counter stage outputs being connectable in different manners to produce different repetitive signal sequences, said counter in response to the receipt of an interrogation signal following transmission of the last binary signal of said sequence being reset and not responsive for transmitting a binary signal until receipt of the next interrogation signal, a detector means aT said central station for analyzing said transmitted signal sequence to determine the existence of an alarm condition at said remote station, said detector means including a multistage binary counter having an input responsive to the transmission of an interrogation signal for incrementing said counter and a plurality of counter stage outputs for facilitating generation of a second repetitive sequence of signals related to said first sequence, said counter stage outputs being connectable in different manners to produce different repetitive signal sequences related to said different repetitive signal sequences of said remote station counter, and counter synchronization means at said central station for resetting said central station counter in response to the failure of said remote station to transmit a width-modulated signal following receipt of an interrogation signal, thereby synchronizing said remote and central station counters.
 8. The system of claim 7 wherein said remote station detector includes signal-sampling means operative in response to receipt of a binary signal for generating a sampling pulse for sampling said transmission line means to determine the nature of the binary signal transmitted, whereby the analysis of the received signals is independent of variations in transmission line delays.
 9. The system of claim 1 wherein said first and second transmitting means collectively include a multistage binary counter having an input responsive to the receipt of an interrogation signal for incrementing said counter and having a plurality of counter stage outputs connectable for facilitating transmission of a first repetitive sequence of width-modulated binary signals, said outputs being connectable in different manners to produce different repetitive signal sequences, said counter in response to the receipt of an interrogation signal following transmission of the last binary signal of said sequence being reset and not responsive for transmitting a binary signal until receipt of the next interrogation signal, and wherein said detector means at said central station includes a multistage binary counter having an input responsive to the transmission of an interrogation signal for incrementing said counter and a plurality of counter stage outputs for facilitating generation of a second repetitive sequence of signals related to said first sequence, said counter stage outputs being connectable in different signal sequences related to said different signal sequences of said remote station counter, and wherein said central station further includes counter synchronization means for resetting said central station counter in response to the failure of said remote station to transmit a width-modulated signal following receipt of an interrogation signal, thereby synchronizing said remote and central station counters.
 10. A system for monitoring at a central station the condition, alarm or nonalarm, of an alarm condition sensor located at a remote station, said system comprising: a code generator at said remote station for generating a normal sequence of binary signals, when said alarm condition sensor is in said nonalarm condition, and for generating an alarm sequence of binary signals when said sensor is in said alarm condition, interrogation means at said central station for transmitting interrogation signals to said remote station, said interrogation signals being transmitted at least at the rate of one interrogation signal per binary signal transmitted by said remote station, transmitting means at said remote station responsive to said interrogation signals for causing said code generator to transmit signals of either said alarm or nonalarm sequence, each of said signals of said transmitted sequence being transmitted in response to receipt of a separate interrogation signal, randomizing means for randomly varying the time interval between successive binary signal transmissions, and detector means at said central station for analyzing said traNsmitted signal sequence to determine the existence of an alarm condition at said remote station.
 11. The system of claim 10 wherein said central station interrogation means includes a delay circuit for initiating said interrogation signals following receipt of transmitted binary signals after delay intervals, and wherein said randomizing means includes means associated with said delay circuit for randomly varying said delay intervals and thereby randomly varying the time interval between successive binary signal transmissions.
 12. The system of claim 10 wherein said code generator includes a multistage binary counter having an input responsive to the receipt of successive interrogation signals for successively incrementing said counter and having a plurality of outputs from said stages connectable for facilitating transmission of a first sequence of binary signals when successively incremented, said counter stages being connectable in different manners to produce different signal sequences, and wherein said detector means includes a multistage binary counter having an input responsive to the transmission of an interrogation signal for incrementing said counter and having a plurality of counter stage outputs for facilitating generation of a second sequence of signals related to said first sequence, said detector counter stages being connectable in different manners to produce different signal sequences related to said first signal sequences of said remote station, and signal-sequence-changing means activated at said central station for similarly altering the interconnection of said remote station counter outputs and the interconnection of said central station counter outputs to thereby similarly alter the binary signal composition of said remote and central station binary signal sequences.
 13. The system of claim 1 further including randomizing means for randomly varying the time interval between successive binary signal transmissions.
 14. The system of claim 13 wherein said central station interrogation means includes a delay circuit for initiating said interrogation signals following receipt of transmitted binary signals after delay intervals, and wherein said randomizing means includes means associated with said delay circuit for randomly varying said delay intervals and thereby randomly varying the time interval between successive binary signal transmissions.
 15. A system for individually monitoring at a central station the condition, alarm or nonalarm, of a plurality of alarm condition sensors located at a remote station, said system comprising: first transmitting means at said remote station which are responsive to each of said alarm condition sensors for transmitting to said central station a normal sequence of width-modulated binary signals when none of said alarm condition sensors is in an alarm condition and for transmitting to said central station an alarm sequence of width-modulated binary signals when one of said sensors is in an alarm condition, said transmitting means including multistage binary counting means having input means responsive to the receipt of successive interrogation signals for successively incrementing said counting means and having a plurality of outputs from said stages connectable for facilitating transmission of said sequences of signals when successively incremented, said stages being connectable in different manners to produce different signal sequences, second transmitting means at said remote station which are responsive to each of said sensors for transmitting to said central station a sensor identity sequence of width-modulated binary signals when none of said sensors is in an alarm condition, said sensor identity signal sequence having at least as many signals as there are sensors to be individually monitored with each sensor associated with a different signal, said second transmitting means operative to transmit an inversion of a sensor-associated signal when said associated sensor is in an alarm coNdition, means at said central station for interrogating said first and second remote station transmitting means prior to the transmission of each width-modulated signal, first detecting means at said central station for analyzing the signals transmitted by said first transmitting means to determine if said alarm signal sequence was transmitted, second detecting means at said central station for analyzing the signals transmitted by said second transmitting means to determine if said inversion of a sensor-associated signal was transmitted, said detecting means including multistage counting means having an input responsive to the transmission of an interrogation signal for incrementing said counter and having a plurality of counter stage outputs for generation of a signal sequence related to said normal signal sequence, and counter outputs being connectable in different manners to produce different signal sequences related to said different signal sequences of said remote station, a plurality of alarm indicators each corresponding to a different one of said remote station sensors, and each responsive to both said first and second detectors, each said indicator being activated to alert central station personnel that its respective remote station sensor is in an alarm condition in response to both the detection by said first detecting means of the transmission of said alarm signal sequence and the detection of said second detecting means of the transmission of an inversion of its respective sensor-associated signal, and signal-sequence-changing means activated at said central station for similarly altering the interconnection of said remote station counter outputs and the interconnection of said central station counter outputs to thereby similarly alter the binary signal composition of said remote and central station binary signal sequences.
 16. The system of claim 10 wherein said transmitting and interrogating means include carrier frequency signal generators and wherein the output of said code generator frequency modulates the output of said transmitting carrier frequency signal oscillator.
 17. The system of claim 1 further including a third detecting means for analyzing the signals transmitted by said first transmitting means to determine if at least one but less than all of said first signals are inverted, said third detecting means providing an output signal upon inversion of at least one but less than all of said first signals, a circuit fault indicator responsive to said output of said third detecting means for alerting central station personnel to the existence of a fault condition when said output of said third detecting means if produced for a specified interval indicating multiple inversions of at least one but less than all of said first signals.
 18. The system of claim 6 further including means at said remote station for selectively enabling and disabling said sensor, a second code generator at said remote station for generating different sensor status signals indicating the enabled and disabled status of said sensor, said status signals being input to said transmitting means for transmission to said central station, sensor status detector means at said central station responsive to said transmitted status signals for indicating a change in status of said sensor, and means at said central station for transmitting to said remote station a signal indicating detection by said status detector of a change in status of said sensor, thereby alerting remote station personnel to the fact that central station personnel are aware of the changed status of said sensor. In a system having a fault detector and a fault indicator responsive to said detector for indicating the existence of a fault, a signalling circuit for signalling failure of said fault-responsive indicator, said signalling circuit comprising: a transistor having its emitter-collector path in series with said fault indicator and its base responsive to said fault detector for allowing current from a source to flow through and activate said fault indicator in response to an output from said fault detector accompanying the detection of a fault, said transistor being biased to conduct insufficiently to activate said fault indicator in the absence of said fault detector output, resistive means connected to pass said current flow through said indicator, a signal device responsive to the voltage across said resistive means for activating said signal device when said voltage is zero, due to interruption of said base, failure of said source, or failure of said fault indicator, said signal device remaining unactivated in response to the existence of voltages across said resistive means due to said current flow and said insufficient transistor conduction. 